Thermoplastic molding techniques for ceramic powders include, for example, injection molding, blow molding, compression molding, transfer molding, drawing, rolling and extrusion. In thermoplastic ceramic extrusion, a heated mixture of a ceramic powder and a thermoplastic binder are forced through a die to form shapes like bars, rods, tubing or fibers. The thermoplastic binder must be suitable as a vehicle for the ceramic powder, maintain the extruded shape, and be easily removed from the extruded shape, leaving behind a shaped powder compact. Binder removal is normally done by thermal decomposition. After binder removal, the shaped compact is baked to sinter the ceramic powder into a densified body. The ceramic powder and thermoplastic binder mixture are herein referred to as a thermoplastic ceramic molding composition.
A major problem with the binder for thermoplastic ceramic molding compositions has been that thermal decomposition of the binder, from the shaped powder compact, generally introduces defects, such as cracks, pits and voids into the sintered body. U.S. Pat. Nos. 4,571,414 and 4,551,496 disclose a thermoplastic ceramic molding composition of a sinterable ceramic powder mixed with an organic acid and a thermoplastic co-polymer of ethylene and vinyl acetate. The thermoplastic binder of the '414 and '496 patents can be formed by the techniques described above to form polycrystalline ceramic articles such as gas turbine air foils, crucibles, thin-walled hollow tubes, long rods, spherical bodies and nozzles. The binder can be removed by thermal decomposition without leaving service-limiting cracks, voids or other defects in the ceramic body.
Ceramic fibers are increasingly being utilized in ceramic composites and metal matrix composites to increase toughness, strength, thermal shock resistance, impact resistance, and other desirable properties in the composite. In some instances, very fine diameter continuous ceramic fibers or filaments will theoretically provide a greater increase in the strength, toughness or other desirable properties in such composites. As the diameter of a ceramic fiber decreases it becomes increasingly difficult to thermoplastically extrude so that, as a practical matter, thermoplastic extrusion can be used to form ceramic fibers as small as about 250 microns in diameter.
Heretofore, ceramic whiskers have effectively been utilized to provide improvements in the strength and toughness of various composite bodies. Ceramic whiskers are very fine diameter single crystal filaments as fine as 0.1 micron in diameter and are characterized further by a very small size, typically about 10-100 microns in length.
Single crystal or polycrystalline continuous ceramic fibers or filaments having a very fine diameter can provide further improvements in the strength, toughness, volume fraction of fiber loading, fiber alignment and other desirable properties as well as provide improvements in the manufacturing processes for both the composites and fibers. However, at the present time, a very limited number of processes for making specific ceramic materials into very fine diameter ceramic fibers or filaments are known. For example, single crystal sapphire filaments have been grown from an alumina melt by slowly drawing a filament from the melt.
Therefore, it is an object of this invention to provide a binder system suitable for fabrication of a wide range of continuous ceramic fibers, and very fine diameter continuous ceramic fibers or filaments. As used herein, a continuous fiber does not necessarily refer to an infinite length but rather a length of fiber suitable for use in a variety of manufactured articles such as a composite.
It is also an object of this invention to provide a thermoplastic ceramic molding composition suitable for extrusion into elongate ceramic bodies and containing a binder that can be substantially removed from the ceramic body by thermal decomposition of the binder to provide a sintered ceramic body essentially free of the cracks, pits or voids sometimes formed by the decomposition of binders.
Another object is an extrusion process for forming continuous elongate ceramic bodies.
A further object is a process for forming fine diameter continuous ceramic fibers or filaments that have a finer diameter than can be produced by thermoplastic ceramic extrusion alone.
An additional object is an elongate ceramic body formed from the thermoplastic ceramic molding composition, that can be uniformly elongated until the cross-section of the ceramic body is reduced to as small as 25 microns.